This invention relates to a medical apparatus for enhancing and for correcting skeletal mechanics. More specifically, this invention relates to the correction of certain bone alignment deformities that impair optimal biped mechanics of the foot.
Excessive pronation (hyperpronation) of the foot leads to abnormal motion to the first metatarsal resulting in excessive strain on the soft tissues supporting this bone. After a prolonged period of these excessive forces the soft tissues will stretch out and no longer be able to support the first metatarsal. This instability leads to an abnormal deviation of the first metatarsal bone resulting in foot pathology. The deforming forces acting on the first metatarsal leads to three possible deviations: (1) pure medial deviation of the metatarsal, (2) dorsal deviation (dorsiflexion) of the metatarsal, and (3) the combination of the previous two, dorsomedial deviation. When the first metatarsal deviates medially (toward the body's middle/away from the little toe) the hallux (big toe) will deviate laterally (toward the little toe). When the first metatarsal deviates dorsally (up), the proximal phalanx of the hallux plantarflexes (angles down) and the distal phalanx angles dorsally. A dorsomedially deviated first metatarsal forces the hallux to deviate in a plantar-lateral direction.
In addition to the soft tissue laxity of the first metatarsocuneiform joint there exists another deforming force. Several tendons from the leg and foot attach to the hallux. When the muscles from these tendons contract to move the hallux, this will lead to a retrograde force on the first metatarsal. The generated force from this action will further contribute to the deviation of the first metatarsal bone. The end result is that there will be instability at both the proximal and distal joints of the first metatarsal bone. To reiterate, the problem is not with the metatarsal bone itself but with the joints at the ends of this bone.
Yet, another complicating factor in the formation of first metatarsal bone (or “first ray”) deformities is the pathoanatomy of the end of the first or medial cuneiform bone. In a normal foot, the joint between the first metatarsal and first cuneiform should be straight across from medial to lateral. A common finding with first ray deformities is that the distal end of the cuneiform (the end in contact with the first metatarsal) is deformed. The end of a deformed cuneiform will commonly angle medially. In other words, the distal lateral portion of the cuneiform is longer than the distal medial portion of the bone. This deformation causes instability of the first metatarsal bone and contributes to the medial shift of the metatarsal bone.
The deviation of the first metatarsal bone leads to the formation of a bunion (hallux valgus) and can also lead to other deformities of the first ray. These other deformities include metatarsus primus elevatus, metatarsus primus varus, hallux abductovalgus, hallux limitus, hallux rigidus and metatarsus primus adductus. With all of these deformities, there is usually no actual intrinsic deformity of the first metatarsal bone itself. The deformities are proximal at the first metatarsal cuneiform joint or distal at the first metatarsophalangeal joint.
The current treatment of the deformity of the first ray ranges from conservative non-surgical to aggressive surgical procedures. Non-surgical treatment includes the use of an arch support, supportive shoes, taping and strapping, padding, etc. Multiple surgical procedures have been described for the realignment of the first metatarsal bone to the cuneiform and the proximal phalanx of the hallux. These osseous (or bone surgical) procedures include cutting and shifting of the first metatarsal bone into a more rectus (or straight) position and fusing the base of the first metatarsal bone to the first cuneiform.
The problem with non-surgical treatment options is that it is ineffective in eliminating the causative factor. Also, every step leads to further deformity of the metatarsal bone. Because the problem with this deformity is intrinsic, usually external remedies are ineffective in controlling the deforming forces.
Surgical remedies consist of various osseous procedures to realign the metatarsal bone to the hallux. These osseous procedures of the first metatarsal bone only provide for a cosmetic effect while the instability of the first metatarsal/first cuneiform joint (the metatarsocuneiform joint) still exists. These types of procedures straighten the metatarsal bone with respect to the hallux but leave instability at the first metatarsocuneiform joint. Since the instability at the first metatarsocuneiform joint still exists, the first metatarsal bone will eventually deviate again and lead to the occurrence of an overall foot deformity.
Another surgical procedure to correct this deformity of the first metatarsal bone has been suggested. The procedure involves, inserting either an opening wedge or a bone graft in the metatarsocuneiform joint. Still another method is to fuse the first metatarsal to the first cuneiform via arthrodesis (the fusion of two bones by surgical procedure or otherwise). These procedures lead to long recovery periods, at least six months, and can fail. Wedges can displace from the fusion site and bone grafts can fail at a rate of as much as 20-30%.
Another procedure is the shortening of the first metatarsal bone. Unfortunately, this transfers the body's weight to the head of the second metatarsal bone instead of the first. Often, further pathology ensues such as callus formation under the ball of the foot leading to further pain and possible ulceration. In normal ambulation, the weight of the body lands on the outer aspect of the heel and is transferred to and through the foot ending up through the first metatarsal bone. The second metatarsal head is not meant to take the weight of the body and it is possible for it to develop a stress fracture. This improper weight redistribution can lead to fractures in other parts of the foot.
A final set of procedures use plates of various shapes to stabilize the first metatarsal/first cuneiform joint while arthrodesis of these bones occurs. The problem with these procedures is that the plates mimic a screw or staple and stabilize the fusion site between the two bones. Consequently, there is still a shortening of the first metatarsal bone leading to the possibility of other ill effects. Moreover, these plates are rather bulky and usually have to be removed after bone arthrodesis is achieved.